In the metal strip painting and coating industries, it has long been the practice to pass painted, coated or inked metal strip through elongated horizontal curing ovens, in which the strip hangs in a relatively flat catenary curve from rollers set at the entrance and exit ends of the oven. Alternatively, the strip may be suspended within the oven by an air flotation system or a combination of catenary and flotation systems as shown in U.S. Pat. No. 4,242,087.
As the strip travels through the oven in one direction, heated air is forcibly circulated within the oven so that convection heat transfer rapidly dries and cures the paint, ink or other material applied to the strip. This is to be contrasted with furnaces which usually have radiant heating with no more than natural circulation of the atmosphere within the furnace. Curing ovens for metal strip are currently constructed of sheet metal and are typically as much as ten feet high by ten feet wide, or more, in transverse cross section, and from approximately 100 to 200 feet or more in length. The generally rectangular tube shape of current ovens require considerable internal and external bracing for maintaining the integrity of the ceiling and walls of the oven chambers.
A problem associated with current curing ovens is that the internal bracing creates angled surfaces which collect paint particles, fume condensation, and other debris, increasing the frequency of cleanings required for the oven. The internal bracing is additionally more difficult to clean than the other surfaces of the ovens.
Another problem arising in present rectangular ovens arises from non-uniform temperature distribution and airflow within the oven. This may create safety hazards in that flammable materials accumulate where temperatures are low or the air flow is stagnant. The paints and inks used in metal strip painting and coating contain flammable organic solvents. The flammability of these solvents require that the temperature and composition of the air in the oven be closely controlled. For most paint and ink coatings, it is desirable to maintain a constant air temperature of about 500.degree. F. Eddies and vortices caused by corners of the oven, however, cause pressure and temperature differentials to occur within the oven. This results in unwanted overheated and cool areas to occur within the oven.
Overheated areas of an oven may reach temperatures sufficient to reach the flash point of the solvents evaporating from the painted metal strip. When this happens an explosion may occur.
The internal corners of a forced circulation oven create undesirable cool and stagnant areas. Cool air enters the oven, particularly when an oven is operated at a slight negative pressure. Vortexes due to corners and internal structure in the oven may cause substantial temperature differences to occur in the oven, particularly in corner areas which have poor air circulation, and which may inherently be cooler anyway. Cooler air entrained in the main circulation of air in the oven may enhance temperature non-uniformities in the strip as well.
The cooler areas of the oven may result in the condensation of fumes evaporated from the coating materials, creating a fire hazard. Many coating materials include volatile components which condense on the walls of the oven where cool. Deposits of sticky dust build up in such areas Such deposits may accumulate to the point where pieces fall off, leaving blemishes on the coated surfaces being cured. The convection ovens, therefore, require frequent cleaning. This is a difficult, unpleasant and time consuming task. Manways are provided through the walls of the oven for people to enter and wash down the walls. Although the deposits usually wash off with blasts of water, considerable time can be wasted during cleaning. Further, to save time during cleaning, people enter the oven while it is still quite warm, which is both unpleasant and hazardous.
It is, therefore, desirable to provide means for not only minimizing condensation of undesirable deposits within the oven, but also to facilitate cleaning when deposits do occur.